1. Field of the Invention
The present invention relates generally to bichromatic spectrophotometers, i.e., to instruments which measure a characteristic of a sample by comparing the extent to which light at two different wavelengths is transmitted through, or absorbed by, the sample. More particularly, the invention relates to means for selecting the two wavelengths.
2. Description of the Prior Art
The usual application of a bichromatic spectrophotometer is to determine the concentration of a specific chemical in a liquid sample. A "sample" wavelength and a "reference" wavelength are chosen so that the amounts of light transmitted through the sample at those wavelengths are substantially dependent and independent, respectively, of the concentration of the chemical. The spectrophotometer's measurement of the relative amounts of light transmission at these two wavelengths thus indicates the chemical concentration.
In a spectrophotometer, a common method of establishing the wavelength of light transmitted through the sample is by using a broadband light source in combination with a narrowband filter. This allows the wavelength to be selected by selecting the filter. In a bichromatic spectrophotometer, a pair of optical filters commonly is mounted in a single filter holder so that the mounting arrangement determines which of the two filters is the "sample" filter and which is the "reference" filter.
A bichromatic spectrophotometer may comprise a broadband light source which simultaneously illuminates the sample at both the "sample" and "reference" wavelengths and two photosensors respectively mounted relative to a "sample" and a "reference" filter so as to continuously measure the light transmission at the "sample" and "reference" wavelengths, respectively.
More commonly, a bichromatic spectrophotometer comprises only a single photosensor which periodically alternates between measuring the light transmission at the "sample" and "reference" wavelengths. The alternation means typically comprises "sample" and "reference" filters mounted in a rotating disc optically aligned with a stationary aperture, or else comprises the two filters mounted in a stationary filter holder optically aligned with a rotating disc having one or more apertures. An example of the latter arrangement is disclosed in U.S. Pat. No. 3,690,772 to Endl. In addition, the spectrophotometer typically comprises means for producing electrical synchronizing signals which indicate which of the two wavelengths of light is being measured by the photosensor at any moment in time and a demultiplexer or demodulator circuit for directing the photosensor output to a "sample channel" amplifier and a "reference channel" amplifier when the synchronizing signals indicate the photosensor is measuring light transmission at the "sample" and "reference" wavelengths, respectively.
A bichromatic spectrophotometer is commonly accompanied by a large assortment of filter pairs mounted in one or more filter holders so that the user can select the combination of sample and reference wavelengths according to the particular chemical under analysis.
One common shortcoming with prior art bichromatic spectrophotometers is that they cannot reverse the roles of the two filters in a mounted filter pair as the sample and reference filters, respectively. Therefore, two separate mounted filter pairs are required to make both combinations available. For example, suppose a filter holder contains a filter pair in which the sample filter has a 500 nanometer (nm) wavelength and the reference filter has a 600 nm wavelength. If a user requires a 600 nm sample filter and a 500 nm reference filter, the above filter pair will not satisfy the requirement. Instead, a second filter pair having a 600 nm sample filter and a 500 nm reference filter must be mechanically substituted.